(1) Field of the Invention
The present invention relates to calcined zeolites, which are aluminosilicates, with uniform intracrystal mesopores. In particular, the present invention relates to a process for forming the zeolites by reacting porogens comprising a modified silane, wherein a polymer is covalently linked to silicon in the silane, with a silica source and an alumina source. The present invention also relates to novel porogens. In particular, the present invention relates to calcined zeolite crystals with uniform intracrystal pores of between about 1 and 10 nm in one dimension.
Further, the present invention relates to a process for forming the zeolites by the use of a Si covalently linked organic polymer silane as a porogen in the formation of the zeolite.
(2) Description of the Related Art
Zeolites are crystalline aluminosilicates that are widely used in industry as catalysts and molecular sieves (Corma, A. Chemical Review 97, 2373-2419 (1997)). These materials have crystalline open framework structures with well defined micropore windows that range in size from 0.3 to 1.2 nm, depending on the type of framework structure. Molecules smaller than the pore openings are capable of entering the framework pores. The small framework micropores limit the usefulness of zeolites for catalytic reactions and adsorption processes when the size of the guest molecule is larger than the pore size of the zeolite. Even when the guest molecule is smaller than the framework pore size of the zeolite, the diffusion rate of reactants and products into and out of the channels can be slow, thereby limiting catalytic activity and selectivity.
Open framework structures with pore sizes of a few nanometers, ie., <10 nm, in dimensions are expected to function as size or shape selective catalysts for the conversions of large molecules. For instance, using non-zeolitic mesostructured aluminosilicates for the catalytic cracking of polymeric macromolecules, (Aguado et al., Energy and Fuels 1997, 11, 1225-1231) have shown that uniform mesopores a few nanometers in diameter provide higher yields of liquid fuels than are obtained using the same aluminosilicate composition with much larger and less uniform mesopore distributions. Thus, zeolites with uniform small mesopores or uniform large micropores can be expected to function as selective catalysts for the cracking of large petroleum molecules and other catalytic conversions of large molecules. However, zeolites with these desired pore size properties are unknown. Moreover, the kinds of shape-selective mesostructured aluminosilicates studied by Aguado et al. lack atomic order (i.e. the materials are amorphous) and therefore, lack the desire acidity and hydrothermal stability needed for the efficient cracking of large molecules.
There have been many attempts to provide for zeolites with uniform large micropores (1.2-2.0 nm) or small mesopores (2-10 nm). Efforts to crystallize stable zeolites with framework pores larger than about 1.2 nm have been unsuccessful. Consequently, attention has been directed at introducing textural mesoporosity into known zeolite structures with conventional small framework micropores. Textural porosity is distinct from framework porosity, because it is independent of the crystal structure of the zeolite. The introduction of textural porosity in zeolite compositions has been achieved in many ways. As will become evident from the discussion below; however, the methods disclosed thus far for the introduction of textural porosity in zeolites inevitably results in very broad textural pore size distributions and with average textural pore sizes (>10 nm) that are far larger than is desired for selective large molecule adsorption and catalysis.
One (1) approach to achieving textural pores in zeolites is to make the fundamental crystal size of the zeolite very small, preferably less than 100 nm. Nanosized zeolites can be synthesized in low yields in the form of clear solutions by kinetic control of the crystallization process (Bein, T., et al., Angewandte Chemie-International Edition 41(14), 2558-2561 (2002); Martens, J. A., et al., Angewandte Chemie-International Edition 40 (14), 2637-2640 (2001)), (Martens, J. A., et al., Journal of Physical Chemistry 103(24), 4972-4978 (1999)). If the nanoparticles are subjected to nano-filtration or ultracentrifugation, and isolated in powdered form, the voids formed between the nanoparticles represent textural mesopores. However, the interparticle mesopores have a very broad pore size distribution and the average textural pore size is much larger than 10 nm. The broad mesopore distribution limits and even precludes product or reactant selectivity based on molecular sizes or shape. The value of interparticle mesopores between nanoparticles lies primarily in speeding-up reaction rates through improved molecular diffusion through the framework pores of the crystals. For example, nanosized zeolites have been observed to show higher catalytic performance than conventional monolithic zeolites due to the larger external surface areas and the more rapid diffusion of reactants and products through crystals that are typically smaller than a few hundred nanometers in size. (Yamamura, M., et al., Zeolites 14, 643-649 (1994); Vogel, B., et al., Catalysis Letters, 79, 107-112 (2002); Landau, M. V., et al., Industrial & Engineering Chemistry Research 42, 2773-2782 (2003); and Zhang, P. Q., et al., Catalysis Letters 92, 63-68 (2004)).
Aggregated or intergrown zeolites nanoparticles with interparticle mesopores can be produced by growing the crystals in the nanopores of a carbon template, a process also as zeolite nanocasting. For instance, nanocrystalline ZSM-5 with different crystal sizes were synthesized in the confined spaces provided by a carbon black matrix (Schmidt, I., et al., Inorganic Chemistry 39(11), 2279-2283 (2000); Jacobsen, C. J. H., et al., Chemical Communication 8, 673-674 (1999)). Also, colloid-imprinted carbons have been used as a template for the nanocasting of aggregated and intergrown nano-sized ZSM-5 with fundamental crystal sizes from 12 to 90 nm (Kim et al., Chemical Communication 15, 1664-1668 (2003)). The resulting nanosized zeolite exhibited mesopores that arise from voids between aggregated and intergrown nanocrystals. Although the fundamental crystal size of the resulting nanocasted zeolite is very small, the packing or aggregation of the crystallizes is non-uniform, resulting in interparticle mesopores that also are non-uniform and larger on average than the size of the fundamental particles themselves. In a related approach using carbon aerogel as a template, Tao and co-workers (PCT International Application WO 2003104148; Tao, Y., et al., Journal of the American Chemical Society 125(20), 6044-6045 (2003)) synthesized ZSM-5 with an average textural mesopore size centered at 11 nm and a width at half height of 3 nm. This is the smallest and narrowest interparticle textural mesoporosity yet reported for a crystalline zeolite.
There are several disadvantages to the nanocasting approach to the formation of interparticle mesoporous zeolites. Firstly, little or no mesoporosity is generated below 5 nm, the size regime most desired for size and shape of selective chemical catalysis. Another disadvantage of carbon templating methods to obtaining nanosized mesoporous zeolites is the need to provide for nanosized carbon templates for forming the nanoparticles and then destroying the carbon template through calcination.
Another strategy to obtaining mesoporous zeolites involves the formation of mesopores within, as opposed to between, zeolite crystals. The intracrystal textural mesoporous zeolites can be achieved by steaming and chemical leaching of monolithic zeolites crystals to remove zeolite mass, leaving behind mesopores. (Groen, J. C., et al., Chemistry-A European Journal 11(17), 4983-4994 (2005); Groen, J. C., et al., Microporous and Mesoporous Materials 87 (2), 153-161 (2005)). These techniques usually produce intracrystal pores much larger than 10 nm. Also, the resultant pores are not especially uniform in diameter.
In yet another approach to the formation of intracrystal mesopores, attention had been focused on the incorporation of carbon nanoparticles into zeolite crystals as they crystallize. The subsequent removal of the occluded carbon particles by calcination results in intracrystal mesopores that replicate the size and shape of the templating carbon. In particular, carbon black particles have been used as a templating agent to form intracrystal mesopores in zeolites (Jacobsen, C. J. H., et al., Journal of the American Chemical Society 122(29), 7116-7117 (2000); Janssen, A, H., et al., Microporous and Mesoporous Materials 65(1), 59-75 (2003)). Carbon nanotubes also have been used as nanoparticle templates for the formation of intracrystal mesopores (Schmidt, I., et al., Chemistry of Materials 13(12), 4416-4418 (2001)). But due to the weak interaction between carbon and a silica matrix, the nanoparticle carbon templates were often extruded out of the zeolite crystal during crystallization, resulting in nanosized zeolite products with interparticle mesoporosity rather than intraparticle mesoporosity. Even when carbon nanoparticles were successfully occluded into the zeolite crystals using special gel processing techniques, the mesopore size distribution could be no better than the particle size fidelity of the occluded carbon nanotubes, which are notorious for forming aggregates with a broad size distribution.
There are two (2) major disadvantages associated with the above approaches to the formation of zeolites with textural mesoporous. The first one is that, with the exception of the zeolite formed by nanocasting in a carbon aerogel (c.f., Tao et al.), the resulting mesopores, whether inter- or intra-particle mesopores, typically are widely distributed in size. The uniformity of mesopore is dependent on the nanocrystal size or the carbon porogens, both of which tend to be irregular in both size and shape. Therefore, the resultant zeolites reflect the same broad distribution of mesopores, which is not suitable for shape or size selective catalytic conversions. Furthermore, even when the mesopore size distributions is comparatively narrow as in the case of zeolites formed through the use of carbon aerogels as templates (c.f., Tao et al), there is little or no pore volume or large micropore range is provided in the 1-2 nm large micropore range or in the 2-10 nm small mesopore domain, which is highly desired for shape or size selective conversions or separations of large molecules. Moreover, little or no textural mesoporosity has yet to be disclosed in the size range below 6 nm, where such mesoporosity is expected to show shape-selectivity in catalytic reactions such as petroleum cracking and refining.
Mesostructured aluminosilicates with ordered networks of uniform pores in the mesopore ranging from 2˜50 nm have been considered as potential substitutes for large pore zeolites. However, due to the absence of atomic order in the mesostructured framework walls, these compositions lack the desired hydrothermal stability and acidity for such applications. In an effort to combine the features of zeolites and mesostructured aluminosilicates, van Bekkum and co-workers reported a double-template approach for the synthesis of zeolite—mesostructured aluminosilicate composites. (van Bekkum, H., et al., Chemical Communication 2281-2282 (1997); van Bekkum, H., et al., Chemistry of Materials 13, 683-687 (2001)). Also, Kaliaguine and co-workers used coating and post-synthesis crystallization techniques to form composite mixtures of zeolite nanocrystals dispersed in a mesoporous aluminosilicate support with amorphous framework walls (Kaliaguine, S., et al., Angewandte Chemie-International Edition 40(17), 3248-3251 (2001); Kaliaguine, S., et al., Angewandte Chemie-International Edition 41(6), 1036-1040 (2002)). These composite compositions provide uniform mesoporosity in combination with a zeolite phase, but the phase contributing the mesoporosity is not a zeolite. Thus, dispersing zeolite nanoparticles on mesostructured aluminosilicate supports also does not provide the zeolitic porosity in the desired large micropore to small mesopore range for the selective chemical catalysis of large molecules.
Organosilanes containing small organo groups have been previously incorporated into zeolite structures for the purpose of modifying the chemical or physical surface properties of the zeolite (Yan, Y. et al., Microporous Mesoporous Materials, 17(15), 347-356 (2005); Tatsumi, T. et al. Chemistry of Materials, 17(15), 3913-3920 2005)). Also, organosilanes have been used to prepare nanosized zeolite having increased outer surface area (Aguado, J. et al. WO 2005026050). However, due to the small size of the silane organo group or the inappropriate ratio of polymer weight to silane modifier used in these studies, intra-crystalline textural porosity was neither observed nor anticipated.